Digital halftoning for the purpose of generating colour images with multiple levels is a well known art. For an extensive review of the field of halftoning, reference is made to "Digital Halftoning" by R. Ulichney, MIT Press, Cambridge, Mass. The area of halftoning is generally divided into the fields of dithering and error diffusion. The field of dithering is generally divided into the areas of random dither, clustered dot ordered dither and dispersed dot dither. Of these, clustered dot dither and dispersed dot dither are the most prevalent.
Both clustered dot dither and dispersed dot dither are based on the creation of a threshold screen pattern or dither matrix, for example an 8.times.8 array of image pixels. This dither matrix is compared, pixel by pixel, with inputted digital image values, with the dither matrix being treated in a `modulo` or `wrap-around` sense. If the inputted value is greater than or equal to a current dither matrix value, the output at that point is set to be "on" and if it is less than the output, it is set to be "off".
The difference between the two techniques of clustered dot dither and dispersed dot dither is that in clustered dot dithering the lower threshold values are centred in a clustered pattern. Whereas in the dispersed dot dithering screen pattern the lower threshold values are scattered. As such, the clustered-dot technique has a central dot that increases in size as the signal level increases whilst the dispersed-dot technique has small scattered dots that increase in number as the signal level increases. In both techniques, the number of levels that can normally be represented is equal to the size of the dither matrix. For example, an 8.times.8 dither matrix is generally designed to produce 64 unique levels. If more levels are required then larger dither matrix sizes can be used. However, for a given type of display equipment. this will mean a reduction in the effective spatial resolution that can be achieved with clustered dot dithering because the dither matrix size will be larger, thereby taking up more space for the printing of each individual pixel. By way of example, high quality offset printers are generally able to print approximately 2400 dots per inch. However, images are normally printed in the order of 150 pixels to the inch. Since 2400/150=16, this provides a 16.times.16 array of dots for each individual pixel, and allows for 256 different possible intensity levels.